1. Field of the Invention
The present invention relates to a multi-access transmission method and multi-access transmission apparatus for use in an SDH (Synchronous Digital Hierarchy) communication network.
2. Description of the Prior Art
In these days, communication traffic is increased on the Internet, and communication providers or ISPs (Internet Service Providers) face a significant problem that how the increased traffic is incorporated into existing networks.
Most of the communication traffic on the Internet is non-voice data and has burst traffic patterns. In other words, it is rarely that actual communication always occupies allocated bands on a communication line used. Nevertheless, existing networks (for example, point-to-point dedicated lines) are currently used, and thus bands can not be necessarily used efficiently. As a result, a secondary problem occurs that the cost of communication paths is high.
Most of data transferred on the Internet is important data for commercial use. High reliability and quality as well as reduced cost are required for transmission networks. In addition, with a recent wider variety of services, requirements for reliability and quality vary among services. Furthermore, demand grows for application to WANs (Wide Area Networks) establishing connection among LANs (Local Area Networks) and for broadcast or multicast.
With these demands as a background, the aforementioned requirements are difficult to sufficiently meet with techniques for transmission networks or LANs typified by a conventional synchronous digital hierarchy.
When the conventional networks are used for connection among LANs, as shown in FIG. 1A, connections between respective sites require that SDH paths 120 are provided in a mesh type point-to-point form. This method provides individual dedicated lines shown as bold lines in FIG. 1 between respective points, i.e. between A point and B point, A point and C point, A point and D point, B point and C point, B point and D point, and C point and D point. This requires a significant number of SDH paths (that is, dedicated lines).
Another method exists for providing SDH paths in a hop-by-hop type point-to-point form as shown in FIG. 1B. The method provides SDH paths (dedicated lines) 121 between respective points, i.e. between E-F, F-G, G-H, and H-E and arranges routers for routing at respective points of E, F, G and H. For example, when data is transmitted from E point to G point, the data is once transmitted from E point to F point and then from F point to G point with outing at F point. In such a transmission form, SDH paths 121 are sectioned at respective nodes to complicate the management of SDH paths 121.
In this manner, in the conventional transmission system for connection among LANs shown in FIGS. 1A and 1B, since the SDH paths are connected in a point-to-point form and one SDH path is allocated to one communication path, bands of an SDH path are always occupied by a single communication path whether or not all of the bands of the SDH path are used for actual communication. For example, when communication is performed connecting A point to B point as shown in FIG. 2, SDH path 40 connecting A point to B point is provided and occupied by communication between A-B points in a conventional transmission apparatus. As a result, the bands of SDH path 40 are always provided for communication between A-B points. No problem occurs when all the bands of SDH path 40 are used for communication between A-B points. However, some unused bands, if any, could not be effectively used. Therefore, more bands are required for one communication path, resulting in inability to efficiently use potential bands of a transmission line and to provide an inexpensive communication line in terms of cost.
Additionally, conventional LAN techniques have problems such as difficulty of rapid recovery from faults, and do not necessarily provide a network with high reliability and quality. Solutions for application to wide area networks (WANs), and for broadcast and multicast are also not sufficient.
In JP, A, 07-170238, proposed is an add/drop multiplexer apparatus for exchanging signals between an arbitrary main channel and an arbitrary dependent channel with a small signal exchange capacity. In the add/drop multiplexer apparatus, add/drop multiplexer units are respectively provided for four different dependent connection ports in a communication network including an SDH transmission line having four main channels. Only one pair of the four main channels passes through a time slot mutual exchange unit possessed by the add/drop multiplexer unit, while the remaining main channels bypasses it. Another pair of the main channels passes through another time slot exchange unit. However, also in the add/drop multiplexer apparatus, when communication is performed between two different points, the communication between the two points occupies an SDH transmission line, thereby making it impossible to effectively use unused bands in total bands of the SDH transmission line, if any.
In JP, A, 08-008949, proposed is an SDH2-fiber ring optical multiplexing apparatus for selectively limiting the number of protection channels especially at the occurrence of a communication failure to increase available bands at normal operation. Specifically, a transmit/receive control unit for controlling transmission/reception of a predetermined channel signal allocates different number of channels such that the number of protection channels is limited with respect to the number of working channels on an optical fiber transmission path for increasing available bands at normal operation of a network. However, the optical multiplexing apparatus increases available bands only when a communication failure occurs. Additionally, also in the optical multiplexing apparatus, an SDH transmission line is basically occupied by communication between two points, thereby failing to effectively utilize unused bands in total bands of the SDH transmission line.
In JP, A, 09-511624 which corresponds to WO95/20846, proposed is an SDH add/drop multiplexer provided with a bypass and arranged to interface to a high bit rate transmission system through the bypass in use. Also in this add/drop multiplexer, as is the case with the apparatus disclosed in the aforementioned JP, A, 07-170238, when communication is performed between two different points, an SDH transmission line is occupied by the communication between the two points. For this reason, unused bands in total bands of the SDH transmission line, if any, can not be effectively used.
In addition, JP, A, 10-224390 proposes a signal switching method for addressing failures on a network. The signal switching method involves branching a signal to be switched into a plurality of signals A at a transmitting end and then transmitting the signals, receiving the signals A as a plurality of reception signals at a receiving end, and switching to a signal of the plurality of reception signals that is close to normal. A through signal not subjected to drop is processed in one package in a closed manner, while only a signal to be dropped is transferred to a package on the drop side for switching. For a signal to be added, a signal is branched from a package on the add side with a backboard and sent to another route. Also with the signal switching method, when communication is performed between two different points, an SDH transmission line is basically occupied by the communication between the two points. As a result, unused bands in total bands of the SDH transmission line, if any, can not be effectively used.
It is a first object of the present invention to provide a multi-access method capable of effectively using unused bands in total bands of an SDH path, if any, when communication is performed between two different points through the SDH path.
It is a second object of the present invention to provide a multi-access apparatus capable of effectively using unused bands in total bands of an SDH path, if any, when communication is performed between two different points through the SDH path.
In the present invention, when multi-access is performed to a synchronous digital hierarchy path, packets or cells are multiplexed in the synchronous digital hierarchy path to share one synchronous digital hierarchy path among a plurality of communications. This enables effective and efficient use of bands of the synchronous digital hierarchy path.
In the present invention, term xe2x80x9cSDHxe2x80x9d or xe2x80x9csynchronous digital hierarchyxe2x80x9d, includes any kinds of digital hierarchy of synchronous type. For example, xe2x80x9csynchronous digital hierarchyxe2x80x9d to which the present invention can be applied includes synchronous digital hierarchy standardized by ITU-T, SONET (Synchronous Optical Network), and synchronous digital hierarchy defined by Bellcore GR-253. Further, term xe2x80x9cSDH pathxe2x80x9d is used as its widest meaning and an SDH path can be used for a dedicated line as well as for a path for multiplexed packets.
The above and other objects, features, and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate examples of preferred embodiments of the present invention.